Liquid substances applied to the airfoil surfaces of aircraft to remove ice in inclement weather are well-known and are important to ensure safe and proper take off of aircraft in winter. It is also well-known that aircraft departures are often delayed under such conditions and the de-icing formulations must often be reapplied if the plane must wait an extended period and the ice reforms. These liquids must also be stable not only through temperature extremes, but also during pumping and spraying (application to the wings) and the taxiing phases of the take off procedure and also must adhere to the wing surfaces during taxiing and ground winds. The fluids may also be diluted somewhat through intentional dilution with water, such as during particularly cold conditions below the expected operating temperatures of the fluids to enable their application, or incidentally as through freezing rain conditions. In addition to these demands, the de-icing materials must also exhibit Newtonian rheology, that is, a viscosity that is shear-independent and time-independent. They simply flow off the wing surfaces after deicing. Such compositions are also useful in the de-icing of other surfaces besides air foils, for example, run-way and vehicle roadway surfaces.
De-icing/anti-icing can be performed in one or two steps. One-step de-icing/anti-icing is carried out with a de-icing fluid or an appropriately diluted or heated anti-icing fluid. The fluid is used to de-ice the aircraft remains on aircraft surfaces to provide limited anti-ice capability. Two-step de-icing/anti-icing consists of two distinct steps. The first step (de-icing) is followed by the second step (anti-icing) as a separate fluid application. Anti-icing fluid is applied to protect the relevant surfaces thus providing maximum possible anti-ice capability. However, usually two different trucks and attendant systems are used to apply the de-icing fluid and then the anti-icing fluid. It takes a few minutes to move the first, de-icing truck away and the second, anti-icing truck into position. Holdover times are thus important for de-icing fluids because the de-icing fluid should remain on the wing to protect it until the anti-icing fluid is applied. If the de-icing fluid does not protect the surface during this period, under some weather conditions clear ice can form in the interim which is quite undesirable. For more information, see SAE Aerospace Recommended Practice: Issued Proposed Draft (SAE ARP4737).
It is important to keep in mind the difference between aircraft de-icing, which is the procedure by which frost, ice, or snow is removed from the aircraft in order to provide clean surfaces, and aircraft anti-icing, which is the procedure which provides protection against the formation of frost or ice and accumulation of snow or slush on clean surfaces of the aircraft for a limited period of time. While there are similarities between de-icing compositions and anti-icing compositions, there are some important differences, too. For example, de-icing compositions have Newtonian rheology. The shear rate of a Newtonian fluid is directly proportional to the shear stress. The fluid will begin to move immediately upon application of a stress; it has no yield stress which must be accomplished before flow begins. One publication describing the important differences between de-icing compositions and anti-icing compositions (including those mentioned above) is M. S. Jarrell, "SAE Type I Aircraft Deicing/Anti-icing Fluids," FAA International Conference on Airplane Ground De-icing, May 28-29, 1992, Reston, VA.
Various anti-icing and de-icing compositions are known. For example, U.S. Pat. No. 4,585,571 mentions de-icing compositions for use on airplanes, runways, streets and the like, which have (a) an alkylene polyol, (b) an anionic surfactant capable of forming a hydrophobic monolayer on the metal surfaces of the aircraft, (c) a hydrophilic wetting agent which is capable of associating with the hydrophobic monolayer, and (d) a coupling agent, which facilitates the association between the wetting agent and the monolayer.
The precipitation of salts from glycol-based fluids when mixed with water is prevented by the addition of copolymers or terpolymers of acrylic acid or alkali metal salt thereof and one or more of the monomers selected from the group consisting of 2-acrylamido-2-methylpropyl sulfonic acid or metal salt thereof, 4-styrenesulfonic acid or metal salt thereof, a lower alkyl hydroxy acrylate, and mixtures thereof, according to U.S. Pat. No. 5,118,434. The glycol-based fluids are mentioned as de-icing fluids.
Chemical Abstract 108:223486p (1988) of Romanian patent document RO 92,551 briefly sets out sprayable de-icing compositions that form films with high shear resistance on aircraft which contain 250-500 parts propylene glycol, 5-15 parts polyacrylamide, 0.5-1 part Na.sub.2 HPO.sub.4 or borax, 0.5-1 part ethoxylated nonylphenol or oleyl alcohol with a degree of ethoxylation of 17, and 250-500 parts water.
U.S. Pat. No. 4,744,913 describes an anti-icing and de-icing agent, based on glycols and water and having crosslinked acrylic polymers useful as a thickener, and also containing customary corrosion inhibitors, surfactants belonging to the group of alkali metal alkylarylsulfonates and a neutralizing agent to adjust the pH to a basic value. The agent contains, as the thickener, two selected crosslinked acrylic polymers in a specific ratio by weight to one another, namely a selected crosslinked acrylic acid or alkali metal acrylate homopolymer and a selected crosslinked acrylic acid/acrylamide or alkali metal acrylate/acrylamide copolymer in a ratio by weight of 2:1 to 10:1. The neutralizing agent may be three compounds, each in a selected amount, namely ammonia, monoethanolamine, diethanolamine and/or triethanolamine as the first alkaline compound, potassium hydroxide as the second alkaline compound and a further alkali metal hydroxide as the third alkaline compound. The new agent was found to exhibit the particular advantage that it has a relatively low viscosity even at arctic temperatures and low shear rates, which ensures rapid and complete runoff of the agent at the take-off of the aircraft even under extreme conditions.
Another liquid agent for anti-icing and protecting against icing-up is described in U.S. Pat. No. 4,358,389, by means of which it is possible, in particular, to free the metal surface of aircraft rapidly and completely from ice, hoar-frost, snow and the like, and to protect the surface against further build-up for a relatively long period. The agent is essentially composed of several components, namely of (a) glycols, (b) water, (c) thickeners, (d) substances insoluble in water, (e) surface-active agents, (f) corrosion inhibitors and (g) alkaline compounds. The quantities are very specific in each case, the quantity of the components (a) and (b) being at least 94%, relative to the total weight of the agent. Its pH value is 7.5 to 10. The component (c) thickeners are crosslinked polyacrylates described in the patent in further detail and also in U.S. Pat. No. 2,923,692.
U.S. Pat. No. 4,954,279 describes a composition consisting of a micro-emulsion of oil in a water/glycol solution containing certain thickening agents (e.g. natural and artificial gums, cellulose ethers, carboxymethylcellulose and hydroxyethylcellulose), emulsifiers, substantially water-insoluble, partially polar oils, and alkanolamines to provide the desirable viscosity and shear stability needed to give a composition having effective de-icer and anti-icing properties. The emulsifier may be anionic surfactants, nonionic surfactants and mixtures thereof, such as organo-phosphates, phosphonates, sulfates, sulfonates, fatty acid salts, alcohols, phenols, amines, fatty acids and their alkoxylated derivatives.
A composition for accelerating the melting of layers of snow or ice using anionic, nonionic and cationic wetting agents in the absence of other melt-accelerating agents is described in U.S. Pat. No. 3,412,030. The anionic wetting agent may be an alkyl aryl sulfonate, having at least one alkyl group having 12 to 15 carbon atoms. The nonionic wetting agent is selected from the group consisting of alkyl oxyethers and esters and thioethers and esters having the formula: EQU RA(CH.sub.2 CH.sub.2 O).sub.x --CH.sub.2 CH.sub.2 OH
where A is selected from the group consisting of ethereal oxygen and sulfur, amine, carboxylic ester and thiocarboxylic ester groups, R is a saturated or unsaturated hydrocarbon group of 8 to 18 carbon atoms or an aralkyl group of 8 to 18 carbon atoms, and x is from 1-20 or an alkyl.
British Patent Specification 1,272,464 discusses a de-icing fluid having an aqueous solution of an alcohol component which comprises one or more of ethylene glycol, propylene glycol and glycerol; a polyethylene glycol ether of a diisoalkylphenol; and a thickening agent which is a copolymer of acrylic acid or methacrylic acid.
There remains a need for aircraft de-icing compositions that exhibit Newtonian rheological properties and that retain their properties as the composition is diluted with water. The de-icing fluids should also remain stable during pumping and/or spraying, and should be physically stable on a surface such as an airfoil for relatively long periods of time to delay or avoid re-application of the compositions during departure holdovers, particularly during freezing rain, or generally humid and cold conditions, for example.